[chore] Weed out dead code buildup, add weeding step to sanitize-pr.

[elland]

As a first step, this adds weeding to the sanitize-pr step, next step would be making it part of our CI linting step.

I also sneaked in a small change around using verification code subsystems, we've had the effect for a while, but it was unused.

Checklist

• [x] Add a new entry in an appropriate subdirectory of changelog.d
• [x] Read and follow the PR guidelines